A “d­warf plan­et” or­bit­ing our sun three times fur­ther away than the dis­tant dwarf plan­et Plu­to
is around the same size as that better-known, frig­id world, as­tro­no­mers have found.

Sci­en­tists took the mea­sure­ments when the fur­ther-off world, Er­is, made a rare oc­culta­t­ion—in which it passes in front of a dis­tant star, dim­ming its light slight­ly. Such an event pro­vides a means of de­ter­min­ing Er­is’ size and im­prov­ing our un­der­stand­ing of the
ob­ject, ac­cord­ing to the re­searchers.

The in­ves­ti­ga­t­ion may al­so, they said, help ex­plain the un­usu­al bright­ness of Er­is, which was dis­cov­ered in 2005 and or­bits the Sun about 10 bil­lion km (six bil­lion miles) away on av­er­age.

Eris, named after a god­dess of Greek myth­o­logy, has a moon named
after her daugh­ter, Dys­nom­ia. Ow­ing to the great dis­tances, meas­ur­ing the size or de­tect­ing a pu­ta­tive at­mos­phere
for Eris is dif­fi­cult.

Af­ter the oc­culta­t­ion was seen in No­vem­ber 2010, Bru­no Si­cardy of the Pierre and Ma­rie Cu­rie Uni­vers­ity and Ob­serv­a­to­ry of Par­is and col­leagues meas­ured Er­is as about 2,326 km (2,011 miles) wide.

Plu­to, which was of­fi­cially re­clas­si­fied as a dwarf plan­et in 2006 af­ter be­ing con­sid­ered a plan­et for dec­ades, is an es­ti­mat­ed 2,300 to 2,400 km wide. Those es­ti­mat­ed dis­tances would make Earth five to six times wid­er than ei­ther dwarf plan­et.

As ex­pected, Er­is is round or nearly so, the usu­al shape for larg­er ce­les­tial ob­jects, said Sicardy and col­leagues. They al­so sug­gest its bright sur­face may be caused by a col­lapsed at­mos­phere, fro­zen by its cold en­vi­ron­ment. Er­is might de­vel­op an at­mos­phere si­m­i­lar to that of Plu­to
when it or­bits clos­er to the Sun, they spec­u­lat­ed.

The find­ings are to ap­pear in the Oct. 27 is­sue of the re­search jour­nal
Na­ture.

A “dwarf planet” orbiting our sun three times further away than the distant dwarf planet Pluto around the same size as that better-known, frigid world, astronomers have found.
Scientists took the measurements when the further-off dwarf planet, Eris, made a rare occultation—in which it passes in front of a distant star, dimming its light slightly. Such an event provides a means of determining Eris’ size and improving our understanding of the planet, according to the resaerchers.
The investigation may also, they said, help explain the unusual brightness of Eris, which was discovered in 2005 and orbits the Sun about 10 billion km (six billion miles) away on average.
Owing to the great distance, measuring the size or detecting a putative atmosphere is difficult.
After the occultation was seen in November 2010, Bruno Sicardy of the Pierre and Marie Curie University and Observatory of Paris and colleagues measured Eris as about 2,326 km (2,011 miles) wide.
Pluto, which was officially reclassified as a dwarf planet in 2006 after being considered a planet for decades, is an estimated 2,300 to 2,400 km wide. Those estimated distances would make Earth five to six times wider than either dwarf planet.
As expected, Eris is round or nearly so, the usual shape for larger celestial objects, said Sicardy and colleagues. They also suggest its bright surface may be caused by a collapsed atmosphere, frozen by its cold environment. Eris might develop an atmosphere similar to that of Pluto as it orbits closer to the Sun, they speculated.
The findings are to appear in the Oct. 27 issue of the research journal Nature.